Electrometric studies on uranyl molybdates as a function of pH

Estudos eletrométricos sobre a formação de molibdatos de uranila em função do pH

Electrometric studies on uranyl molybdates as a function of pH

Shiva PRASAD^*

Jaqueline Viana BARROS^*

ABSTRACT: The formation and composition of uranyl molybdates obtained by the interaction of uranyl acetate and sodium molybdate at specific pH levels 7.6, 5.5 and 4.1 have been studied by employing electrometric techniques involving pH and conductometric titrations. The results provide cogent evidence for the formation of three uranyl molybdates having the molecular formulae UO₂O.MoO₃, 3UO₂O.7MoO₃ and 2UO₂O.8MoO₃ in the vicinity of pH 5.7, 4.6 and 3.8, respectively. Analytical investigations of the compounds have also been carried out which substantiate the results of electrometric study.

KEYWORDS: Molybdates; uranyl; electrometric study.

Introduction

The chemistry of molybdenum is very prominent in both biogical and industrial systems.^4,16 Recent studies have shown that certain molybdates have antiviral, including anti-AIDS, and antitumor activity.⁶ Altough a large number of studies have been done in the field of molybdate chemistry, the chemical state of isopolymolybdates, obtained on acidification of a molybdate solution, is not well understood because of the complexity in polymerization. Jander et al. claimed existence of Mo₃O₁₁^4-, HMo₃O₁₁^3-, HMo₆O₂₁^5-, H₂Mo₆O₂₁^4-, H₃Mo₆O₂₁^3-, H₇Mo₁₂O₄₁^3-, H₇Mo₂₄O₇₈^5- and H₉Mo₂₄O₇₈^3- from diffusion and optical experiments.⁵ Bye claimed the existence of Mo₇O₂₄^6-, Mo₆O₂₀^4-, Mo₄O₁₃^2- and HMo₆O₂₀^3- by cryoscopic study.³ In 1959, Sasaki et al. deduced from potentiometry that the main complex formed is Mo₇O₂₄^{. 6-13} Subsequently mathematical analysis was applied to potentiometric equilibrium curves, and Sasaki et al. claimed the existence of Mo₇O₂₄^6-, HMo₇O₂₄^5-, H₂Mo₇O₂₄^4- and H₃Mo₇O₂₄^3-up to a value of Z (average number of H⁺ being consumed by MoO4^2-) of around 1.4.^14,15 Aveston et al.¹ by centrifuge data could only tell that in the range studied, the species probably contain more than 6 and less than 9 Mo atoms. Sasaki et al. proposed the presence of large isopolymolybdate anions of the order of 20 Mo in the solution of Z > 1.5¹⁴. Numerous species such as HMoO₄^-, H₂MoO₄, Mo₂O₇^2-, HMo₃O₁₁^3-_, Mo₆O₁₉^2-, Mo₇O₂₄^6-, HMo₇O₂₄^5-, H₂Mo₇O₂₄^4-, Mo₈O₂₆^4-, HMo₈O₂₆^3-, Mo₁₂O₃₇^2-, H₇Mo₂₄O₇₈^5-, Mo₃₆O₁₁₂^8-, etc. have been reported in many recent publications.^7,8

On account of the complexity of the relation of equilibria between the polyanions or due to the experimental difficulty in early works, the conclusions of earlier workers seem to be overstrained and hence it was considered worthwhile to make a careful and precise study of the acid-molybdate system by electrometric techniques, which have provided more conclusive evidence on the condensation process of vanadate, antimonate, thiotungstate and tunsgtate anions.⁹ In earlier publications the author has reported the effect of pH change on a solution of Na₂MoO₄ and composition of mercuric molybdates.^11,12 In view of the interesting results obtained, it was considered worthwhile to investigate the composition of uranyl molybdates obtained by the action of uranyl cations on different molybdate anions at specific pH levels by means of the electrometric techniques.

Materials and methods

Na₂MoO₄.2H₂O, (CH₃COO)₂UO₂.2H₂O, Na₂B₄O₇.10H₂O and hydrochloric acid of extra-pure grade were used, and their solutions were prepared in deionized distilled water. Concentration of sodium molybdate solutions was further verified by determining molybdenum with oxine as MoO₂(C₉H₆ON)₂.^18a The hydrochloric acid solutions were standardized with recrystallized sodium tetraborate decahydrate.^18b

The pH-meter, conductometer, electrodes and titration cells were used as reported earlier.¹⁰ 25 ml of the titre solution was placed in the cell each time and thermostated at 25± 0.1^oC. Using different concentrations of the reactants, a series of glass electrode and conductometric titrations was performed. The observed pH changes were plotted as a function of volume of titrant added. The inflections obtained by the curves were confirmed by the pronounced maxima in dpH/dV graphs. The breaks in the conductometric titrations were located by plotting corrected conductance as a function of volume of titrant added. The same concentrations of reactants were employed in the two techniques for the sake of comparison of results. The pH and conductometric titration curves are plotted together in the same Figure for similar reasons and also for the sake of brevity. Only a limited number of experimental points are shown in the curves to maintain legibility and to avoid overlapping. The electrometric titration results for the formation of uranyl molybdates are summarized in Tables 1.

The precipitates obtained at the end-points of titrations between uranyl acetate and sodium molybdates were also analyzed to substantiate the electrometric results. The different uranyl molybdates were prepared by mixing stoichiometric amounts of mercuric nitrate solution with the respective sodium molybdate solutions. The precipitates obtained were washed several times with aqueous 20% (v/v) ethanolic solution and dried in a vacuum dessicator for 40 h. A known amount (2 g) of each of the above precipitates was dissolved in sulfuric acid and then analyzed quantitatively for molybdenum by precipitating with a-benzoinoxime and weighing as molybdic oxide.^18c Uranium was determined from the filtrate with oxine.^18d From the proportion of uranium and molybdenum in the compounds thus obtained their composition were established. The results are summarized in Table 2.

Results and Discussion

The solution of Na₂MoO₄ was prepared in deionized distilled water. Sodium paramolybdate (pH 5.5) and octamolybdate (pH 4.1), as reported by recent studies,^11,12 were prepared by progressive additions of hydrochloric acid to Na₂MoO₄ solutions in the molar ratios 8H:7Mo and 3H:2Mo, respectively.

8HCl + 7Na₂MoO₄ = Na₆Mo₇O₂₄ + 8NaCl + 4H₂O

12HCl + 8Na₂MoO₄ = Na₄Mo₈O₂₆ + 12NaCl + 6H₂O

Uranyl normal molybdate

Figures 1 and 2 illustrate the curves of pH and conductometric titrations performed between the solutions of sodium normal molybdate and uranyl acetate.

In direct titrations (Figure 1, curve 1), when uranyl acetate solution (pH 3.6) was added to the Na₂MoO₄ solution (pH 7.6) a sharp fall in pH was noted with an inflection at the molar ratio of UO₂²⁺:MoO₄^2- as 1:1 in the vicinity of pH 5.7, corresponding to the stoichiometry for the formation of uranyl molybdate, UO₂O.MoO₃. The inflection of the curve is located by the pronounced maxima in dpH/dV graph (Figure 1, curve 2). In case of inverse titrations (Figure 2, curves 1 and 2), when Na₂MoO₄ solution was used as titrant, the pH first changed very slowly, but at the end-point it jumped upwards producing a sharp maxima in dpH/dV corresponding to the formation of the same compound in accordance with the following equation:

(CH₃COO)₂UO₂ + Na₂MoO₄ = UO₂O.MoO₃ + 2CH₃COONa

Employing similar concentrations of the reactants, both direct (Figure 1, curve 3) and reverse (Figure 2, curve 3) conductometric titrations between the solutions of uranyl acetate and Na₂MoO₄ gave well-defined breaks at 1:1 molar ratio of UO₂²⁺: MoO₄^2-, confirming the formation of the uranyl molybdate UO₂O.MoO₃ as suggested by the pH study.

Uranyl paramolybdate

Figures 3 and 4 illustrate the curves of the pH and conductometric titrations performed between the solutions of sodium paramolybdate and uranyl acetate.

In direct titrations, when uranyl acetate solution (pH 3.6) was added from the microburette to Na₆Mo₇O₂₄ solution (pH 5.5), a gradual change in pH was observed till at the stoichiometric end-point (the stage at which the reaction ends if simple double decomposition takes place), a sharp fall in pH (Figure 3, curve 1) and maxima in dpH/df (Figure 3, curve 2) was noted corresponding to the molar ratio of UO₂²⁺: Mo₇O₂₄^6- as 3:1, suggesting the formation of uranyl paramolybdate 3UO₂O.7MoO₃ in the vicinity of pH 4.6. In the case of reverse titrations (Figure 4, curves 1 and 2) when Na₆Mo₇O₂₄ solution was used as titrant, the pH first changes slowly but at the stoichiometric end-point a marked jump in pH and a sharp maxima in dpH/dV was observed, suggesting the formation of the same compound. The reaction can be represented as follows:

3(CH₃COO)₂UO₂ + Na₆Mo₇O₂₄ = (3UO₂O.7MoO₃) + 6CH₃COONa.

Employing similar concentrations of the reactants a series of direct (Figure 3, curve 3) and reverse (Figure 4, curve 3) conductometric titrations was performed between the solutions of uranyl acetate and sodium paramolybdate. The titration curves provide well-defined breaks at a point (see Table 1), where the molar ratio of UO₂²⁺:Mo₇O₂₄^6- is at 3:1, confirming the formation of uranyl paramolybdate as suggested by the pH study.

Uranyl octamolybdate

Figure 5 illustrates the changes occurring in the conductance values during the conductometric titrations between the solutions of uranyl acetate and Na₄Mo₈O₂₆. The direct (curve 1) and reverse (curve 2) titration curves show well-defined breaks at the point where the molar ratio of UO₂²⁺:Mo₈O₂₆^4- is 2:1 which corresponds to the stoichiometry for the formation of uranyl octamolybdate 2UO₂O.8MoO₃, in the neighbourhood of pH 3.8. The reaction can be represented by the following equation:

2(CH₃COO)₂UO₂ + Na₄Mo₈O₂₆ = (2UO₂O.8MoO₃) + 4CH₃COONa.

pH titrations between the solutions of uranyl acetate and sodium octamolybdate did not give dependable results which may be ascribed to the very close values of pH of the two solutions.

It is noted that after each addition of the titrant it takes a little time for the pH and conductance values to become steady. A thorough stirring in the neighbourhood of the equivalence point has a favourable effect. The presence of ethanol (20%) improves the position of the end-points and increases the magnitude of the jump in pH curves, as it decreases the solubility of the precipitates formed and minimises hydrolysis and adsorption.

The precipitates obtained at the end-points of the titrations of uranyl acetate with sodium molybdate at different pH levels were also analyzed by the classical methods for uranium and molybdenum. The results obtained (Table 2) confirm those obtained by the electrometric study. The analytical results obtained were also confirmed by atomic absorption spectroscopy.

The present electrometric and analytical study confirm the formation and precipitation of uranyl normal molybdate UO₂O.MoO₃, paramolybdate 3UO₂O.7MoO₃ and octamolybdate 2UO₂.8MoO₃ in the vicinity of pH 5.7, 4.6 and 3.8, respectively. As the structure of these molecules is not known, they are represented as double compounds, the way usually adopted for such compounds.^2,17

Acknowledgement

The authors wish to express their sincere thanks to the CNPq for financial assistance.

PRASAD, S., BARROS, J.V. Estudos Eletrométricos sobre a Formação de Molibdatos de Uranila em Função do pH. Ecl. Quím. (São Paulo), v.23, p.59-70, 1998.

RESUMO: A formação e composição de molibdatos de uranila obtidos a partir de interação de uranila e molibdato de sódio a específicos níveis de pH 7,6; 5,5 e 4,1 foi investigada por meio de técnicas eletrométricas envolvendo titulações condutométricas e potenciométricas entre os reagentes. Os resultados obtidos fornecem evidências convincentes sobre a formação de três molibdatos de uranila com fómulas moleculares UO₂O.MoO₃ , 3UO₂O.7MoO₃ e 2UO₂O.8MoO₃ nas vizinhanças de pH 5,7 ; 4,6 e 3,8, respectivamente. Também foram realizadas investigações analíticas sobre os precipitados obtidos, que comprovam os resultados dos estudos eletrométricos.

PALAVRAS-CHAVE: Molibdatos; uranila; estudos eletrométricos.

References

1 AVESTON, J., ANACKER, E.W., JOHANSON, J. S. Hydrolysis of molybdenum(VI). Ultracentrifugation, acidity measurements, and Raman spectra of polymolybdates. Inorg. Chem., v.3, p.735, 1964.

2 BRAUER, G. (ed.) Handbook of Preparative Inorganic Chemistry. 2 ed. New York; Academic Press, 1965. v.2, p.1705.

3 BYE, J. Complex equilibria of molybdates. Ann. Chim. Fr., v 20, p.563, 1945; C. r. Acad. Sci. Paris, v.238, p.239, 1954; Bull. Soc. Chim. Fr., p.1023, 1957.

4 HABER, J. The Role of Molybdenum in Catalysis. London: Climax Molybdenum Co., 1981.

5 JANDER, G., JAHR, K. F., HENKESHESHOVEN, W. Z. anorg. Chem., v.194, p.383, 1930. Cited in: MURATA, K., IKEDA, S. Studies on polynuclear molybdates in aqueous solution by laser Raman spectroscopy. Spectrochim. Acta. v.39A, p.787, 1983.

6 KOPF-MAIER, P., KLOPOTKE, T. J. Cancer Res. Clin. Oncol., v.118, p.216, 1992. Cited in Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed. New York; John Wiley and Sons, 1995. v.16, p.962.

7 OZEKI, T. ADACHI, H., IKEDA, S. Estimation of the dissolved structures and condensation reactivities of mononuclear molybdenum(VI) species in solution using the UV-vis absorption spectra and molecular orbital calculation DV-X. Bull. Chem. Soc. Jpn., v.69, p.619, 1996.

8 POPE, M. T. Molybdenum Oxygen Chemistry: Oxides Oxo Complexes and Polyanions. In: LIPPARD, S. J. Progress in Inorganic Chemistry, New York: Interscience, 1991. v.39.

9 PRASAD, S. Electrometric studies on manganese polyvanadates. An. Acad. bras. Ci., v. 53, p.471, 1981; Electrometric investigations on the formation of different antimonate anions as a function of the pH. Bull. Electrochem., v.6, p.163, 1990; Thiotungstate isopolyanions: An electrometric study. Can. J. Chem., v.59, p.563, 1981; Tungstate isopolyanions: An electrometric study. Quim. Nova, v.17, p.31, 1994.

10 __________. Formation of antimonites and thioantimonites of cobalt as a function of the pH. Ecl. Quim., São Paulo, v.20, p.35, 1995.

11 __________. Isopolymolybdates: An electrometric study. X Simp. Bras. Eletroquim. Eletroanal., São Carlos, SP, p.143, 1996.

12 PRASAD, S. GUIMARÃES, T. L. M. Electrometric investigations on the system acid-molybdate and the formation of heavy metal molybdates. Jour. Braz. Chem. Soc., 1998 (in press).

13 SASAKI, Y., LINDEQVIST, I., SILLEN, L. G. On the first equilibrium steps in the acidification of the molybdate ions. J. Inorg. nucl. Chem., v.9, p.93, 1959.

14 SASAKI, Y., SILLEN, L.G. On equilibria in polymolybdate solutions. Acta Chem. Scand., v. 18, p. 1014, 1964.

15 _____________. Ark. Khemi., v.29, p.253, 1967. Cited in: MURATA, K., IKEDA, S. Studies on polynuclear molybdates in aqueous solution by laser Raman spectroscopy. Spectrochim. Acta. v.39A, p.787, 1983.

16 SIMPSON, C.H. Amer. Paint. Coating J., p.66, 1992. Cited in Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed. New York; John Wiley and Sons, 1995. v.16, p.961.

17 STANDEN, A. (Ed.) Kirk-Othmer Encyclopedia of Chemical Technology. 2 ed. New York: Interscience Publishers. 1967. v.13, p.782.

18 VOGEL, A. I. A Textbook of Quantitative Inorganic Analysis. 3 ed. London: Longmans, 1962, p.(a) 508, (b) 238, (c) 507, (d) 540.

Recebido em 20.11.1997.

Aceito em 9.3.1998.

*

Departamento de Engenharia Química - Centro de Ciências e Tecnologia - Universidade Federal da Paraíba - 58l09-970 - Campina Grande - PB - Brazil.

Publication Dates

• Publication in this collection
  05 May 2000
• Date of issue
  1998

History

• Accepted
  09 Mar 1998
• Received
  20 Nov 1997